1. Field of the Invention
This invention relates to a compound resonator type semiconductor laser device having a structure which is effective to attain a stabilized oscillation wavelength.
2. Description of the prior art
Semiconductor laser devices directed toward mass production can attain laser oscillation at a low threshold current level and obtain considerably satisfactory results in characteristics such as the single transverse mode, the single longitudinal mode, durability, etc., but they have problems with regard to a stabilized oscillation wavelength (i.e., the stabilized lonitudinal mode) in that the oscillation wavelength varies continuously or discontinuously depending upon a variation in temperature and/or electric current, resulting in optical output noise which is noticeable when the laser device is exposed to light and/or a reflected laser light (i.e., backlight) from the laser device is incident upon the laser device. In order to eliminate these problems, distributed feedback (DFB) type lasers and compound resonator type lasers have been developed to try to stabilize the oscillation wavelength. However, these laser devices cannot attain a stabilized oscillation wavelength in a wide range of temperature and is insufficient to prevent noise derived from the backlight.
FIG. 8 shows a conventional compound resonator type laser device which comprises an n-substrate 1, an n-cladding layer 2, an active layer 3, a p-cladding layer 4, a p-cap layer 5, a current blocking oxide film 6, a first laser operation area 7 with a resonator length of (L.sub.1 +L.sub.2) having a striped window region, and a second laser operation area 8 with the resonator length of (L.sub.1 +L.sub.3) having a striped window region, thereby effecting an optical interference between these two laser operation areas 7 and 8 to produce a stabilized oscillation wavelength (stabilized longitudinal mode). The interval .DELTA..lambda..sub.1 of the longitudinal mode in the first laser operation area 7 is proportional to .lambda..sub.0.spsb.2 /2n(L.sub.1 +L.sub.2), while the interval .DELTA..lambda..sub.2 of the longitudinal mode in the second laser operation area 8 is proportional to .lambda..sub.0.spsb.2 /2n(L.sub.1 +L.sub.3), wherein .lambda..sub.0 is the oscillation wavelength and n is the refractive index of the active layer 3. Due to the interference between the longitudinal modes in the laser operation areas 7 and 8, a wide interval .LAMBDA.(=.lambda..sub.0.spsb.2 /2n.vertline.L.sub.3 -L.sub.2 .vertline.) of the longitudinal mode is created resulting in stabilized laser oscillation in the longitudinal mode alone around the peak of the gain distribution. However, it is difficult to form the facets with the optimum length of each of L.sub.1, L.sub.2 and L.sub.3 in such a conventional compound resonator type laser device by a cleavage technique, so that the longitudinal mode cannot be stabilized in a wide range of temperature, but it is stabilized ranging in temperature from 5.degree. to 10.degree. C. at the widest. Moreover, a conventional compound resonator type laser device cannot suppress the unstabilized longitudinal mode resulting from backlight therefrom.
On the other hand, semiconductor lasers have been used in the amplitude modulation (AM) format as a light source for optical communication. However, the frequency modulation (FM) format is advantageous over the amplitude modulation (AM) format in attainment of capacious and rapid optical communication, so that frequency modulating semiconductor laser devices which can effect frequency modulation in a wide range and have the great modulation degree are anxiously expected to be established in the field of communication technologies.
FIG. 9 shows a conventional compound resonator used as a frequency modulating semiconductor laser device which comprises a Fabry-Perot resonator type semiconductor laser operation area 100 having a rectilinear resonator therein with the resonator length of (L.sub.1 +L.sub.2), a modulation area 200 having an L-shaped resonator therein with the resonator length of (L.sub.1 +L.sub.3), a separation groove 30 to electrically separate the waveguide in the semiconductor laser operation area 100 from the waveguide in the modulation area 200, and facets 40, 50 and 60 which are formed by a cleavage technique to constitute the Fabry-Perot resonators. The facet 40 is common to both the resonators in the semiconductor laser operation area 100 and in the modulation area 200. When the electric currents I.sub.1 and I.sub.2 flows into the semiconductor laser operation area 100 and the modulation area 200, respectively, laser oscillation is produced as shown by the arrow marks in FIG. 9. A variation of the electric current I.sub.2 flowing into the modulation area 200 allows a continuous variation of the oscillation wavelength with which a laser light in the single longitudinal mode results from the interference between the two resonators. However, this conventional laser device has an extremely limited wavelength modulation range to the extent of tens of .ANG., resulting in a modulation degree of as low as approximately 1 .ANG./mA so that a sufficient modulation effect cannot be attained.